Dropping water table probably set stage for quake last year in Spain.

One of the things people often wonder about earthquakes is whether human activity can play a role in their occurrence. Sometimes that comes from a desire to assign blame, but often it’s related to a bigger question: could we actively trigger small earthquakes to prevent the big, damaging ones from occurring? While that lofty piece of geoengineering may not be feasible (or even possible), it is true that humans can sometimes trigger earthquakes.

Earthquakes are fundamentally controlled by two factors. The first is the movement of rock, such as tectonic plates. This constant, gradual movement is the source of conflict in an active fault zone—one region of rock is being forced past another. If the two blocks simply slid smoothly by each other along the fault surface, this would be a pretty peaceful process. But this is where the second factor comes along—the friction between the blocks. The stress builds up until it’s great enough to overcome that friction, at which point seismic energy is released violently as the blocks catch up on decades' (or centuries') worth of motion in just a few seconds.

For the most part, the accumulating stress that creates this situation is much too large for human activities to make a difference. We can, however, affect the friction that locks up the fault. Hydraulic fracturing, where fluids are pumped into the ground at extremely high pressures to crack rocks that release natural gas and oil, has been shown to do just that in certain situations. Increasing the fluid pressure inside the fault partially de-stabilizes the friction-lock, lowering the stress threshold necessary to trigger an earthquake just enough for one to occur.

The story of the May 11, 2011 earthquake near Lorca, Spain is a different one. This magnitude 5.1 quake occurred at a shallower depth than usual (less than 4 kilometers), leading to surprisingly strong shaking at the surface that caused extensive damage in the city and nine deaths. A study published in Nature Geoscience shows this earthquake was probably related to another geologic phenomenon in the area—unsustainable use of groundwater.

Records show the water table around Lorca has dropped a remarkable 250 meters over the past 50 years. Just as in California’s San Joaquin Valley and the scenic city of Venice, depleting large volumes of groundwater actually causes the land surface to sink. This is because the sediments compact without the water pressure that helps hold spaces between grains open. As a result, the land surface around Lorca is dropping in elevation by as much as 16 centimeters per year.

But that’s only half of the story. The removal of all that water also represents a large removal of weight that was pressing down on the rocks below. And just as a massive ice sheet depresses the land surface—and when it melts away, the surface rebounds—the loss of water causes the rocks to pop up a bit.

To see how all this related to the May 11 earthquake, the researchers turned to satellite data and computer models. The satellites recorded subtle changes in elevation near the fault that occurred during the earthquake. Together with seismometers, this helps scientists work out exactly where the fault slipped, and by how much.

Armed with this information, they used computer models to calculate the effects of the groundwater depletion. The model showed where the depletion altered the stresses along the fault—in exactly the location where the earthquake occurred. That could be a coincidence, but the researchers offer good reasons to think it really is connected.

The removal of the mass of groundwater, and the resulting rebound in the crust, acts to reduce some of the force clamping the fault closed. Like the example of hydraulic fracturing, this lowers the amount of friction that needs to be overcome for an earthquake to occur.

The rebound also created some motion parallel to the fault that probably contributed. To picture this, imagine a pad of paper with one edge placed against a wall. If you make the pad bulge upwards in the middle, the left and right sides will slide along the wall as they are pulled toward the middle.

Critically, all this may also explain why the earthquake was so shallow—and therefore, damaging. This was exactly the depth where the effects of the groundwater depletion were greatest.

It’s not as though an earthquake wouldn’t happen in this location without the groundwater depletion. The stresses that build up and create the conditions where an earthquake can occur have very little to do with human activities. However, it appears that this earthquake occurred where and when it did because of the groundwater depletion.

And that’s what makes this story so important to seismologists. When evaluating the seismic risks in an area, it could be critical to understand how human activities are interacting with the natural processes at work. That’s not an easy thing to do, but it may be necessary. When it comes to preparing for earthquakes, the very last thing you want is a surprise.

I blame doctors: they're always telling us to drink 8 glasses of water per day.

Not to derail the thread, but if people weren't eating processed foods with added salt you probably wouldn't even need 2-3 glasses per day. That's the actual hidden message behind the whole "drink 8 glasses of water per day" (which is ridiculous, by the way).

1. Interferometric synthetic aperture radar can "potentially" measure centimetre-scale changes in deformation over timespans of days to years. My experience with satellite derived DEM data says that happens rarely at that scale. Also the data tends to be in the 30 to 90 meter resolution scale. I would question if the data is good enough to arrive at the conclusion state at the end of the last paragraph: "However, it appears that this earthquake occurred where and when it did because of the groundwater depletion." - I will have to go read the article in Nature Geoscience.

2. The article mentions no evidence of overuse of groundwater as the driving force behind the lowering of the groundwater level. - I will have to go read the article in Nature Geoscience.

Articles like this disappoint me...catchy headlines with very little evidence in the actual article to back up the headline. ARS can do better and we should be more critical of press articles about scientific publications.

1. Interferometric synthetic aperture radar can "potentially" measure centimetre-scale changes in deformation over timespans of days to years. My experience with satellite derived DEM data says that happens rarely at that scale. Also the data tends to be in the 30 to 90 meter resolution scale. I would question if the data is good enough to arrive at the conclusion state at the end of the last paragraph: "However, it appears that this earthquake occurred where and when it did because of the groundwater depletion." - I will have to go read the article in Nature Geoscience.

2. The article mentions no evidence of overuse of groundwater as the driving force behind the lowering of the groundwater level. - I will have to go read the article in Nature Geoscience.

Articles like this disappoint me...catchy headlines with very little evidence in the actual article to back up the headline. ARS can do better and we should be more critical of press articles about scientific publications.

At first glance:1. In this instance they are mapping ground data and measuring the phase change of the return signal. While the actual wavelength of the transmitted carrier wave might (and would) preclude accuracy in the centimeter scale, measuring the return phase shift would increase resolution quite a bit. Properly calibrated it's doable. 2. This article is an overview. Hence the DOI.

Eediot. That won't cause an earthquake. The earth will careen off into space because you've upset its orbital balance. The Chinese tried that years ago. We, in the Western Hemisphere, successfully foiled their evil plan by jumping in the air at the exact moment they did, thusly saving civilization from doom. It must be true, I read it in the National Enquirer while sitting in the Bad Boy Corner at my aunt's.

1. Interferometric synthetic aperture radar can "potentially" measure centimetre-scale changes in deformation over timespans of days to years. My experience with satellite derived DEM data says that happens rarely at that scale. Also the data tends to be in the 30 to 90 meter resolution scale. I would question if the data is good enough to arrive at the conclusion state at the end of the last paragraph: "However, it appears that this earthquake occurred where and when it did because of the groundwater depletion." - I will have to go read the article in Nature Geoscience.

2. The article mentions no evidence of overuse of groundwater as the driving force behind the lowering of the groundwater level. - I will have to go read the article in Nature Geoscience.

Articles like this disappoint me...catchy headlines with very little evidence in the actual article to back up the headline. ARS can do better and we should be more critical of press articles about scientific publications.

1- This is a common application of InSAR- you'll find a lot of seismology studies applying it. I don't know the details of the signal processing, but they do indeed get down to very fine resolutions (sub cm). I take it you have access to the paper behind the paywall, but if not, you can still see a thumbnail of the figures on the landing page- figure 2 shows the inSAR data.

2- The water table drawdown forms a classic cone of depression (again, see Figure 1). The only way to create this feature is through rapid withdrawal.

Eediot. That won't cause an earthquake. The earth will careen off into space because you've upset its orbital balance. The Chinese tried that years ago. We, in the Western Hemisphere, successfully foiled their evil plan by jumping in the air at the exact moment they did, thusly saving civilization from doom. It must be true, I read it in the National Enquirer while sitting in the Bad Boy Corner at my aunt's.

You've only got half the story there... While it's true that the net result of the Western Hemisphere's decision to jump at the exact same time as China was 'zero', the temporarily increased surface compression, and resultant magma compression, nearly lead to the Earth's core to stop rotating. The collapse of the Earth's magnetic field would have led to microwaves toasting pidgins and bridges and such. I know this as I've seen 'The Core'.

Here in the Netherlands we have loads of small earthquakes due to gas mining in the northern part of the country. The gas company usually pays for the damage to houses etc. Sometimes there is a bigger earthquake and then it gets on the national news.

Can We? yes. If we can create the "chunnel" we can do small scale triggers. Should we? Debatable but I lean towards no (for now). I got a small lesson in this from none other than a floor tile installer. He had a crack in the slab running across two sets of tile. There was not enough of the 2nd set of tile to do a full repair job and we had to host some social events in the very near future.

Even though the crack wasn't visible on the 2nd set of tile he basically said if I lay down this crack suppression repair material the portion that is not going to get covered in the 2nd set of tile is guaranteed to get cracked this winter. Sure enough come winter it cracked. thankfully after the holidays.

Even though this was crack suppression and not controlled triggering/releasing of tension we can learn a lesson from this. We need a lot more geophysics large scale model testing before we even try a real attempt at this. If we attempt this then do it in the boonies. Something we do in S.F. may have catastrophic effects in L.A.

I really can't be bothered searching for info on the tubes of the Googles at the moment, but way back in the early days of man, the 1990's or 1980's, the DoD or DoE was performing deep well injections in Nevada, their latest idea at disposing of toxic military (nuclear) waste.

Before too many injection tests had taken place, with direct timing correlation to earthquakes in southern California, someone thought long and hard on it and realized that they were lubricating geologic faults with their deep well injections.

We have been sucking up groundwater like crazy (im in Chicago). The amount of very small earthquakes around here has gone up (or its just we can detect / communicate them better). We really dont need to poke the New Madrid.

1. Interferometric synthetic aperture radar can "potentially" measure centimetre-scale changes in deformation over timespans of days to years. My experience with satellite derived DEM data says that happens rarely at that scale. Also the data tends to be in the 30 to 90 meter resolution scale. I would question if the data is good enough to arrive at the conclusion state at the end of the last paragraph: "However, it appears that this earthquake occurred where and when it did because of the groundwater depletion." - I will have to go read the article in Nature Geoscience.

2. The article mentions no evidence of overuse of groundwater as the driving force behind the lowering of the groundwater level. - I will have to go read the article in Nature Geoscience.

I had similar thoughts reading this.

Quote:

The model showed where the depletion altered the stresses along the fault—in exactly the location where the earthquake occurred. That could be a coincidence, but the researchers offer good reasons to think it really is connected.

While I know absolutely nothing about geology, with the groundwater depleted the area is already more prone to compaction. Wouldn't a non-groundwater-depletion-caused earthquake happening in the region cause a similar level of stress?

The model showed where the depletion altered the stresses along the fault—in exactly the location where the earthquake occurred. That could be a coincidence, but the researchers offer good reasons to think it really is connected.

While I know absolutely nothing about geology, with the groundwater depleted the area is already more prone to compaction. Wouldn't a non-groundwater-depletion-caused earthquake happening in the region cause a similar level of stress?

I'm not sure I understand your question. Let me restate and see if that helps.

They modeled the effects of the groundwater depletion on the fault stresses. So the location where the quake actually occurred is the same location their model said the groundwater depletion was unclamping the fault, etc. (This is in 3-D here, by the way, thinking of the fault as a planar surface.)

Without taking groundwater depletion into account, there's no reason to expect the earthquake to occur at that location on the fault.

We have been sucking up groundwater like crazy (im in Chicago). The amount of very small earthquakes around here has gone up (or its just we can detect / communicate them better). We really dont need to poke the New Madrid.

I'm pretty sure it's far enough away not to matter, but I thought about that, too. The drawdown in Lorca is literally right along the fault.

The model showed where the depletion altered the stresses along the fault—in exactly the location where the earthquake occurred. That could be a coincidence, but the researchers offer good reasons to think it really is connected.

While I know absolutely nothing about geology, with the groundwater depleted the area is already more prone to compaction. Wouldn't a non-groundwater-depletion-caused earthquake happening in the region cause a similar level of stress?

I'm not sure I understand your question. Let me restate and see if that helps.

They modeled the effects of the groundwater depletion on the fault stresses. So the location where the quake actually occurred is the same location their model said the groundwater depletion was unclamping the fault, etc. (This is in 3-D here, by the way, thinking of the fault as a planar surface.)

Without taking groundwater depletion into account, there's no reason to expect the earthquake to occur at that location on the fault.

That helps! I misunderstood what evidence they were using to draw the relationship between the two. Danke!

1. Interferometric synthetic aperture radar can "potentially" measure centimetre-scale changes in deformation over timespans of days to years. My experience with satellite derived DEM data says that happens rarely at that scale. Also the data tends to be in the 30 to 90 meter resolution scale. I would question if the data is good enough to arrive at the conclusion state at the end of the last paragraph: "However, it appears that this earthquake occurred where and when it did because of the groundwater depletion." - I will have to go read the article in Nature Geoscience.

2. The article mentions no evidence of overuse of groundwater as the driving force behind the lowering of the groundwater level. - I will have to go read the article in Nature Geoscience.

Articles like this disappoint me...catchy headlines with very little evidence in the actual article to back up the headline. ARS can do better and we should be more critical of press articles about scientific publications.

At first glance:1. In this instance they are mapping ground data and measuring the phase change of the return signal. While the actual wavelength of the transmitted carrier wave might (and would) preclude accuracy in the centimeter scale, measuring the return phase shift would increase resolution quite a bit. Properly calibrated it's doable. 2. This article is an overview. Hence the DOI.

Ah!! Just saw the DOI. Thanks for the pointer. Still, my objection stands that the article content does NOT support the headline...